Alternate Port Role Shows in STP

I have four physical switches I use for studying the CCNP SWITCH. While labbing spanning-tree protocol, I noticed an RSTP particular port role in IEEE spanning-tree mode. Note that this is not RSTP. My switches are running in 802.1D, STP.

I have redundant links between my switches. I highlighted the text in bold. VLAN 1 is running IEEE STP and you can see F0/1, F0/47, and F0/48 in Altn port role. The switch command spanning-tree mode pvst is shown in the running-config.

The Alternate port role is actually found in RSTP, 802.1W, as you can see in this Cisco document.

It is true that the port roles were introduced with RSTP. However, they definitely make sense for STP (for instance, the difference between an alternate and a backup port is useful to uplinkfast, and it’s also nice to see clearly what the root port is) so we decided to display the role in STP mode also (something we did not do for CatOS afaik).

Now, just for the sake of nitpicking, STP has been in fact deprecated when 802.1w amended 802.1D. An 802.1D switch is an RSTP switch! Running STP nowadays should be in fact running RSTP in STP compatibility mode. And in that case, the switch would definitely display the RSTP roles.

I think that PVST is a sufficient deviation from 802.1D that it makes irrelevant arguing about whether the display of the RSTP role impact the compliance to the standard, so I definitely don’t want to insist on that;-)

Regards,

Francois

So there you have it. TL;DR: IEEE STP on Cisco switches really runs RSTP in STP compatibility mode.